The present invention is directed to providing improved resolution for a surface imaging technique which uses a beam of charged particles and, in particular, for controlling the electron beam of a scanning electron microscope (xe2x80x9cSEMxe2x80x9d) to improve beam alignment in the lower column.
Various instruments are known which rely on emission of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are electron microscopes (e.g., SEMs), focused ion beam microscopes, and mass spectrometers which utilize various well known means to analyze charged particles emitted from the sample.
For facilitating the description of the present invention, it will be explained in connection with an SEM. However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments that steer a beam of charged particles through more than one lens, such as a magnetic lens and an electrostatic lens.
An SEM operates by generating a primary, or incident, electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and have respective trajectories backward along the original beam direction and at angles diverging therefrom. Emitted electrons are collected by a detector, which is arranged above the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is typically processed to create an image of the surface, which is then displayed on a video screen.
An SEM has its main components set up in a part of the apparatus commonly referred to as a column. The column, as the name implies, is usually a vertical arrangement starting at the top with an electron source, or gun, and ending at the bottom with the sample. Positioned between the gun and sample are various well known components that constitute the upper, middle and lower portions of the column and which are used to, for example, correct the shape of the beam, align the beam, and provide scanning by deflecting the beam along an x-direction and a y-direction (see FIG. 3) in a plane perpendicular to the incident beam. SEMs can contain more than one of any of such components, as well as other components that are not discussed herein. Also, it should be understood that the position of the various components need not be as shown in the drawings and/or as described herein, such position being presented for illustrative purposes rather than accuracy.
FIG. 1 shows electron beam 1 passing through a conventional lower column 3 shown in cross section. A typical lower column includes beam deflector 5, scan deflector 7, and electromagnetic lens 11. Deflector 5 is used for aligning the beam within the column. Scan deflector 7 causes beam 1 to controllably depart from its path for a minute range of scanning motion along the x-direction and y-direction (the scanning limit in the x-direction is shown in broken lines, with the deflection angles to points 23 and 25 being exaggerated) in order to scan the surface of sample 9.
Electromagnetic lens 11 is provided for focusing beam 1 to a very small spot on sample 9 to enable high resolution imaging. Such a lens is commonly called an objective lens or a final lens. Its physics and its operation are well known. In the illustrative representation of objective lens 11 in FIG. 1, it includes a toroidal, channel-shaped magnetic polepiece 13 with a lens inner pole 15 and a lens outer pole 17, and a winding 19 inside the channel.
It is well known that a beam directed along the axis of a cylindrically symmetric lens will remain on the axis, i.e., it will not be deflected, as the focus of the lens is changed. If a thin lens model of such a lens were to be applied to simplify the physics involved, and thus facilitate understanding of the invention, even if the beam is not directed along the lens axis but it passes through the lens center it will not be deflected as the focus of the lens is changed. However, there is a degradation of resolution for the case where the beam is not traveling along the optical axis. Similarly, for any optical system, there is a trajectory along which a beam is minimally deflected due to changes in focus. This trajectory, or system axis, will generally not be a simple straight line starting from the electron source and ending perpendicular to the target. The trajectory will have several elements. Although each individual lens might have a well defined axis, these will generally not fall on a single straight line. For charged particle optics systems where the final lens is a compound lens (i.e. comprised of electrostatic and electromagnetic elements) the two lenses have centers that are not generally coincident but, under optimal physical construction, it is possible for the two axes to fall on exactly the same line. In this case it is best to align the beam to this common axis. More generally, as a result of unavoidable imperfections in manufacture and mechanical placement of elements, the two lenses will not have a common optical axis. In this case the optimal alignment (to achieve minimal deflection due to changes in focus) occurs when the beam is directed along the line formed by the two lens centers. When such an alignment is achieved, the image will not shift when the focus of either lens is changedxe2x80x94a critical condition to achieve success with an automated metrology system. Additionally, aligning to the axis of the electrostatic lens is optimal to faithfully reproduce the sample characteristics in an image (especially for achieving image symmetry) while concurrently aligning to the axis of the magnetic lens is optimal for resolution.
Objective lens 11 has an axis identified on the drawing with the unit vector 29 in association with center E. It is necessary to align the path of beam 1 to travel substantially along axis 29 in order to achieve satisfactory performance of the SEM. For example, although it is necessary to vary the focus of objective lens 11 during normal operation of the SEM, unless beam 1 travels essentially along axis 29, such variations in focus induce adverse effects with respect to resolution, distortion, magnification and/or motion of features in the derived image of the sample surface. Thus, deflector 5 is used to align beam 1 with axis 29. Beam 1 remains within the confines of axis 29 even during scanning because the range of motion created by scan deflector 7 is too small and because the alignment of beam 1 with deflector 5 is accomplished by taking into account the scanning motion provided by the scan deflector.
Sample 9 is maintained by voltage source 33 at a predetermined voltage relative to polepiece 13. For example, polepiece 13 can be grounded. The biased sample 9 and the grounded magnetic poles of objective lens 11 form an electrostatic xe2x80x9clensxe2x80x9d. The primary function of this electrostatic lens is to provide a deceleration field for controlling the landing energy of the particles in the beam as they impact on the sample surface. The deceleration field is controlled by adjusting the voltage of source 33. The combination of this electrostatic lens with the above described objective lens constitutes the effective final (compound) lens.
The electrostatic lens has an axis identified on the drawing with the unit vector 31 in association with the center F. Due to the inevitable mechanical misalignments, the electromagnetic axis 29 and the electrostatic axis 31 are typically not coincident nor are they necessarily on a line perpendicular to the sample surface. (The spacing between centers E, F and the difference in orientation between axes 29, 31 are exaggerated for illustrative purposes.) Consequently, merely directing the beam perpendicularly to the sample surface is not necessarily the optimum choice. Thus, if beam 1 travels along axis 29 of objective lens 11, but not along the axis 31 of the electrostatic lens, this misalignment results in an asymmetry in the derived image of the sample surface. The existence of this asymmetry distorts the derived image of the sample, which in turn results in significant errors in any quantitative interpretation of the image (e.g. metrology), thereby significantly and adversely impacting system matching. Further, as was the case with the objective lens, if the electrostatic lens undergoes variations in focus, this will degrade the resolution and cause the other adverse effects mentioned above, such as undesirably changing the magnification. Likewise, if beam 1 travels along axis 31 of the electrostatic lens (but not 29), and the objective lens 11 undergoes variations in focus, this will also have the above-described adverse consequences and will also impair automated attempts at image interpretation.
The magnetic and electrostatic lenses form, in effect, a compound lens with its own axis, which does not necessarily coincide with either one of axes 29 and 31. While it is a relatively simple matter to deflect the beam to the axis of either the objective lens or the electrostatic lens independently, unless the axes of these lenses are coincident, there is no similar means to find the axis of the compound lens. Thus, for optimal system performance, given that image optimization adjustments (e.g. focus) are made to the lenses individually, it is necessary that the beam be directed along the axis of the final (compound) lens.
One object of the present invention is to provide improved operation of a surface imaging apparatus which uses a beam of charged particles.
Another object of the present invention is to provide improved resolution for a surface imaging apparatus which uses a beam of charged particles.
Yet another object of the present invention is to control the electron beam of an SEM, which has a plurality of lenses in the lower column, to avoid adverse impacts induced by changing the focus of one of these lenses.
A further object of the present invention is to control the path of the incident beam of an SEM so that it travels along the axis of a compound lens formed by the objective lens and the electrostatic lens.
These and other objects are attained by one aspect of the present invention directed to a method for imaging a surface of a sample with an apparatus which directs a beam of charged particles at the surface and includes an objective lens and an electrostatic lens for controlling the particle beam to minimize at least one adverse effect induced by changes in operating parameters of the apparatus, the objective and electrostatic lenses forming a compound lens having an axis. The beam is deflected with at least one deflector toward a second deflector, and the beam is deflected with the second deflector toward the sample surface. An optimum location is determined within the second deflector from which the beam can be deflected by the second deflector to substantially travel along the axis of the compound lens. The at least one deflector is set to align the beam with the optimum location, and the second deflector is set to align the beam with the axis of the compound lens.
Another aspect of the invention is directed to an apparatus for imaging a surface of a sample with a beam of charged particles directed at the sample surface and having an objective lens and an electrostatic lens for controlling the particle beam to minimize at least one adverse effect induced by changes in operating parameters of the apparatus, the objective and electrostatic lenses forming a compound lens. A means is provide for generating the beam of charged particles directed at the sample surface. Another means deflects the generated beam with at least one deflector toward a second deflector. Another means deflects the beam with the second deflector toward the sample surface, and another means determines an optimum location within the second deflector from which the beam can be deflected by the second deflector to substantially travel along the axis of the compound lens. Another means sets the at least one deflector to align the beam with the optimum location, and another means sets the second deflector to align the beam with the axis of the compound lens.
Yet another aspect of the present invention is directed to an apparatus for controlling a beam of charged particles used in a machine that images a surface of a sample by directing the particle beam at the surface sample, and which includes an objective lens and an electrostatic lens forming a compound lens, the particle beam being controlled to minimize at least one adverse effect induced by changes in machine operating parameters. A means is provided for deflecting the beam with at least one deflector toward a second deflector. Another means deflects the beam with the second deflector toward the sample surface, and another means determines an optimum location within the second deflector from which the beam can be deflected by the second deflector to substantially travel along the axis of the compound lens. Another means sets the at least one deflector to align the beam with said axis, and another means sets the second deflector to align the beam with the axis of the compound lens.